Statement of problem
Demand is increasing for polyetheretherketone (PEEK) as a fixed dental prosthesis core material. However, information is lacking about how the precision of these restorations is affected by the fabrication procedures.
The purpose of this in vitro study was to evaluate the influence of different fabrication techniques on the marginal precision of PEEK single-crown copings.
Material and methods
A stainless-steel master die was designed to simulate a prepared mandibular second molar to receive ceramic crowns. Thirty PEEK copings were fabricated and divided into 3 groups (n=10) according to the fabrication technique: milled from a prefabricated PEEK blank by using a computer-aided design and computer-aided manufacturing (CAD-CAM) system (PC); pressed from prefabricated PEEK pellets (PP); and pressed from PEEK granules (PG); in addition, 3-mol yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) copings (n=10) were produced by using the same CAD-CAM system and served as a control. Marginal precision measurements (in μm) were recorded at 4 reference points on each coping by using a digital microscope. The data obtained were statistically analyzed by using 1-way ANOVA and the pair-wise Tukey (HSD) test to study the difference between group mean values (α=.05).
The overall mean ±standard deviation marginal gap at the marginal opening for the copings was 78 ±10 μm for PEEK granules copings, 72 ±9 μm for PEEK pellet copings, 45 ±6 μm for PEEK CAD-CAM copings, and 43 ±1 μm for the 3Y-TZP CAD-CAM control. A statistically significant difference was found between the milled and pressed copings as indicated by the ANOVA test ( P<. 001). The pair-wise Tukey honestly significant difference (HSD) test showed a nonsignificant difference ( P >.05) between milled 3Y-TZP and milled PEEK copings; moreover, no significant difference was observed between the PEEK copings pressed from pellets or granules ( P >.05).
The marginal precision of PEEK CAD-CAM–fabricated copings showed significantly lower mean marginal gap values than PEEK pressed copings. The marginal gap mean values recorded were all within a clinically acceptable range (120 μm).
The marginal precision of PEEK restorations is within a clinically acceptable range for all the fabrication techniques tested.
Ceramic restorations are considered an alternative to metal-ceramic restorations because of their excellent esthetics, biocompatibility, high resistance to abrasion, superior mechanical properties, and low thermal conductivity. , The development of materials such as lithium disilicate or 3-mol yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) coupled with innovative processing techniques enable their use in prosthetic dentistry; however, brittleness and the adequacy of the marginal precision have been questioned.
High-strength resins have been introduced as a promising alternative to ceramic materials, including polyaryletherketone (PAEK), polyetheretherketone (PEEK), and polyetherketoneketone (PEKK) materials. PAEK has been used in the aerospace industry for many years and has been used as a bone substitute material for orthopedic surgery involving load-bearing spinal cage devices. The material has good biocompatibility, mechanical stability, and resistance against stress and corrosion. PEEK, the most widely used form of the PAEK resin, is a partially crystalline thermoplastic high-performance polymer (HPP), which is composed of an aromatic backbone molecular chain connected by ketone and ether functional groups. The PEKK material is the most recently developed PAEK resin with 80% higher compressive strength than PEEK materials. PEKK displays both amorphous, used for fixed dental prostheses (FDPs), and crystalline, used for removable partial dentures, material properties, providing unique mechanical, physical, and chemical properties. In addition, PEEK can be used to fabricate interim restorations after implant insertion and for endocrowns. ,
In addition to its excellent biocompatibility, PEEK is highly resistant to thermal degradation with a glass transition temperature of around 143 °C and a melting temperature of approximately 334 °C. The Young modulus of PEEK is approximately 3 to 4 GPa, similar to that of human bone, compared with 200 GPa for zirconia. The flexural strength of PEEK is 140 to 170 MPa, providing restorations with less susceptibility to bulk fractures. PEEK has low thermal conductivity, 0.29 W/mK compared with 2 W/mK for zirconia, so temperature changes in the mouth are not transmitted to the abutment tooth. , In addition, PEEK has low water sorption and low density, 1.28 to 1.32 g/cm 3 compared with 6.49 g/cm 3 for zirconia, enabling the fabrication of light-weight prostheses.
PEEK restorations can be fabricated with the lost wax press technique by using either pellets or granules under standardized pressure, temperature, and time or with the computer-aided design and computer-aided manufacturing (CAD-CAM) technology. However, PEEK has low transparency and a grayish-brown color, so it is not suitable for use as a monolithic restoration in the esthetic region; therefore, the framework must be veneered with a composite resin.
Marginal precision is considered a crucial factor in the success of restorations. Poor fitting margins may lead to cement dissolution, tooth sensitivity, recurrent caries, pulp exposure, and periodontal problems. , The clinically acceptable maximum marginal gap has been reported to be 120 μm, although there is no standard criterion for clinically acceptable marginal precision. Molin et al also reported that marginal gaps of 50 to 100 μm are considered appropriate for successful restorations. Factors that influence the marginal gap of crowns include the fabrication technique, type of abutment, measurements of cemented or noncemented crowns, type of microscope, sample size, finish line configuration, and measurements of the specimens. , ,
Data on the effect of different fabrication techniques on the marginal precision of PEEK restorations are lacking. Therefore, the purpose of this in vitro study was to determine the effect of different fabrication techniques on the marginal precision of PEEK copings. The null hypothesis was that no difference would be found in the marginal precision of PEEK copings produced by different fabrication techniques.
Material and methods
The materials evaluated in this study are presented in Table 1 . A stainless-steel master die was designed to simulate a mandibular second molar prepared to receive a ceramic crown ( Fig. 1 ). The stainless-steel master die was milled to the dimensions of 4.5 mm in height, with a uniform heavy chamfer finish line of 1.0 mm in width and a total angle of convergence of 6 degrees.
|Trade Name||Manufacturer||Composition||Lot No.||Fabrication Technique|
|breCAM. BioHPP||Bredent||80% PEEK with 20% nanoceramic filler||450449||CAD-CAM|
|BioHPP Pellets||Bredent||80% PEEK with 20% nanoceramic filler||441913||Pressing|
|BioHPP Granules||Bredent||80% PEEK with 20% nanoceramic filler||456192||Pressing|
|KATANA Zirconia HT||Kuraray Noritake||97% ZrO 2, 3% Y 2 O 3||DVOVO||CAD-CAM|
|KATANA wax||Kuraray Noritake||45-55% Paraffin wax||NB07||CAD-CAM|
Thirty PEEK copings were divided into 3 groups (n=10) according to the fabrication technique: group PC, milled from a prefabricated PEEK blank by using a CAD-CAM system; group PP, pressed from prefabricated PEEK pellets; and group PG, pressed from PEEK granules ( Fig. 2 ). In addition, 3Y-TZP copings (KATANA HT; Kuraray Noritake Dental Inc) (n=10) were milled by using the same CAD-CAM system and served as a control.
For group PC, the stainless-steel master die was scanned by using a laboratory blue light laser scanner (Dental Wings), and the copings were designed by using a CAD software program (DWOS V.8; Dental Wings) with a 0.5-mm uniform wall thickness and 30-μm virtual cement layer applied 1.0 mm above the finish line ( Fig. 3 ). The data obtained were sent to a 4-axis milling machine (Eco-Mill 50; SHERA Werkstoff-Technologie), where PEEK copings (n=10) were fabricated from a milling blank (Ø 98.5 mm).
For groups PP and PG, wax copings (n=20) were milled with the same dimensions used for scanning, designing, and milling of the previously constructed CAD-CAM PEEK copings. The wax copings were divided into 2 equal groups (n=10) according to the fabrication technique, PP pressed by using PEEK pellets (Ø 20 mm) and PG pressed by using PEEK granules. Sprues with 4-mm diameter and 4-mm length were attached to the wax copings and then invested in a phosphate-bonded investment material (Brevest; Bredent) in a silicone mold according to the manufacturer’s instructions. The silicone mold was heated in a furnace (Vulcan A-130; Dentsply Sirona) to 630 °C for 60 minutes for PEEK granules and 90 minutes for PEEK pellets and left to cool at a cooling rate of 8 °C/minute until 400 °C. Then, the mold was filled with PEEK granules or pellets, and the copings were pressed by using a disposable plunger for each muffle at 0.23-MPa pressure for PEEK granules and 0.45-MPa pressure for PEEK pellets in a vacuum pressing device (For 2 press; Bredent). After cooling, the molds were devested by using 110-μm aluminum oxide particles (Protechno) at 0.2-MPa pressure in an airborne-particle abrasion unit (ESB 2; Eurocem Srl). All copings were evaluated for fit and adjusted by using a silicone polisher (Ceragum Wheel; Bredent) and a polishing paste (Abraso-Starglanz; Bredent). Finally, the copings were steam cleaned for 15 seconds (EGV 18; Eurocem Srl) and left to dry for 10 minutes before testing.
For the control group, 3Y-TZP copings (n=10) were fabricated with the same dimensions used in group PC, except that after designing the coping, the data obtained were enlarged by 20% to compensate for the sintering shrinkage and then sent to the 4-axes milling machine to fabricate 10 copings. The enlarged copings were milled from a presintered 3Y-TZP blank (Ø 98.5 mm, T14 mm) by using tungsten carbide burs Ø 1L and Ø 2L. After milling, the copings were placed in a firing dish and finally sintered to its full density in a special furnace at a temperature of 1500 °C for 10 hours. All copings were examined for deformity and cleaned in a steam cleaner. Subsequently, the copings were adapted to the stainless-steel die until the best precision was achieved. By using electronic calipers (Dial Caliper D; Aura Dental) with a precision of 0.1 mm, the overall thickness of each coping was evaluated at 8 predetermined points representing the midlabial, mesiolabial, distolabial, midlingual, mesiolingual, distolingual, middistal, and midmesial surfaces of the copings.
A digital image analysis system (ImageJ 1.43U; National Institute of Health) in combination with a USB Digital microscope with an integrated camera (Scope Capture Digital Microscope) at ×45 magnification connected to a personal computer was used to measure and evaluate the vertical marginal gap of each coping ( Fig. 4 ). Marginal precision was expressed in pixels; thus, system calibration was performed to convert the pixels into absolute units by comparing a ruler with a scale generated by the ImageJ software.
A spring-loaded holding appliance was developed with a centrally located hole at the base engaging the pointer rod incorporated in the appliance and allowing the die and the coping to be rotated for measurements. In addition, an indentation at the base of the appliance was made by using a fine diamond disk to coincide with the scribed indentations on the stainless-steel die to ensure precise measurements. For all groups, 40 copings were repositioned on the original stainless-steel master die and axially loaded at a force of 30 N in the holding appliance to ensure seating ( Fig. 5 ).
To determine the measuring points on the stainless-steel master die, 4 indentations were made by using a fine diamond disk and inscribed with a fine pencil under a microscope to represent the midlabial, midlingual, middistal, and midmesial surfaces of the die. Measurement at each point was repeated 5 times. The mean values of the marginal gap of the 10 copings in each group for each fabrication technique were recorded and considered to be the gap measurement. The data obtained were collected, tabulated, and subjected to statistical analysis.
Based on the study by Abdullah et al, a sample size of 10 specimens in each group had a 99% power to detect a difference between means of 21.85 with a significance level (α=.05) (2-tailed) and 95% confidence intervals. In 99% (the power) of those experiments, P was <.05 (2-tailed).
Data were presented as mean, standard deviation, or range (minimum-maximum) for values. Data were explored for normality by checking the data distribution and using the Kolmogorov-Smirnov and Shapiro-Wilk tests. One-way ANOVA followed by the pair-wise Tukey honestly significant difference (HSD) tests were used to study the difference among the mean values of the groups (α=.05). A statistical analysis was performed by using a statistical software program (GraphPad Instat; GraphPad, Inc).